Use const Node* in HashMap of DocumentMarkerController

This patch changes key type of HashMap used in DOcumentMarkerController to |const Node*| from |RefPtr<Node>|, which was introduced by r14748(http://src.chromium.org/viewvc/blink?view=revision&revision=14748) to avoid redundant ref counting. 

This patch is a preparation of using |const Node&| parameter in DocumentMarkerController.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/82633002

git-svn-id: svn://svn.chromium.org/blink/trunk@162675 bbb929c8-8fbe-4397-9dbb-9b2b20218538